In U.S. Pat. No. 3,794,468, issued Feb. 26, 1974, in the name of Ronald J. Leonard for "Mass Transfer Device Having a Wound Tubular Diffusion Membrane" and assigned to the assignee of the present invention, a mass transfer device is disclosed. The mass transfer device disclosed in that patent is made by winding a length of hollow tubular conduit of semipermeable membrane about a core to form a wound bundle similar to packages of kite string, in which individual adjacent windings in the same layer are generally parallel to each other, but individual adjacent windings of the conduit in adjacent layers define an angle to each other. The device of that patent is assembled by winding a length of the hollow tubular conduit of semipermeable membrane about a core in a plane which defines an acute angle to the longitudinal axis and intersects both ends of the core. Simultaneously, the core is rotated about its longitudinal axis, or the plane of winding is correspondingly rotated to the same effect, to laterally displace on the core each winding of conduit from its immediately preceding winding. The resulting wound structure allows a low cost construction of a high performance mass transfer device. The flow pattern is around the circumference of the cylindrical element formed by the winding process. However, as the flow goes around the circumference, it will encounter flow paths of different length. Additionally, since the core is rotated at a constant rotational velocity and the angular velocity of the wind is constant, the void fraction of the resulting structure increases radially outwardly of the bundle. In other words, the void fraction near the outside of the bundle will be greater than the void fraction on the inside of the bundle. As used herein, the term "void fraction" connotes the ratio of space to space and fiber, whereby a greater void fraction means more space.
In U.S. application Ser. No. 592,835, filed Mar. 23, 1984, now U.S. Pat. No. 4,576,446, in the names of Ronald J. Leonard and Kenneth M. Johnson for "Bundle Wind System" and assigned to the assignee of the present invention, we pointed out our discovery that performance of the mass transfer device is decreased if the void fraction near the outside of the bundle is substantially greater than the void fraction near the inside of the bundle. We found that an optimum hollow fiber mass transfer device, such as an oxygenator, comprises a bundle in which the flow rate is substantially constant throughout the bundle, the blood outlet saturation is substantially constant throughout the bundle, and the void fraction increases slightly in the radial outward direction of the bundle. In U.S. application Ser. No. 592,835, we disclosed a process for obtaining this optimum device and also disclosed a device having these optimum properties.
The hollow fiber arrangement according to U.S. Pat. No. 3,794,468 and U.S. patent application Ser. No. 592,835 are wound at high length to diameter ratios to limit the fiber loss resulting from crossing over the ends of the bundle. The fiber is cut off to expose the ends of the fiber for gas flow. Length to diameter ratios of approximately four have been found to be practical. However, as the unsupported fiber length increases, it is subject to greater forces due to the blood flow over the fiber. These forces result in a stretching of the fiber due to the elasticity of the fiber, causing the fiber to move out of position in the bundle in the direction of blood flow. Such movement of the fiber is a function of the flow velocity, fluid density, fiber length change per unit length per unit of force per unit of area, fiber inner and outer diameter, and fiber length. The movement of the fiber destroys the precision wind with its controlled variable void volume and causes a migration of fiber toward the outlet of the fiber container device which lowers the void fraction locally in the outlet area. This reduction in void fraction results in a high increase in pressure drop due to the strong dependence of pressure drop on the void fraction. We have discovered a novel process for making a hollow fiber bundle in which this undesirable fiber deflection is reduced.
We have also dicovered that as the fibers move into a fixed volume outlet area, the void fraction decreases. We have discovered a novel mass transfer device which alleviates the outlet void fraction decrease problem.